1. Field of the Invention
The present invention relates to a locking arrangement for secure attachment of a boss to a fixed structural part without the use of fixation bolts, which boss has a central axis extending substantially perpendicular to the plane of the structural part when the parts are assembled.
2. Description of Background Art
Sometimes, structural parts that releasable retain a pipe boss are subjected to bending forces that are so high that the pipe boss is torn off even if it is secured by thick flanges and high strength bolts that are closely placed side by side within a bolt circle. This is in particular true in pipeline installations located on the seabed, i.e. pipes of relatively large dimensions, which are connected to a pipe stub or a pipe boss and are subjected to extreme bending moments.
Traditionally, a pipe boss of the type in question, is designed with an integrated flange, i.e. that it is made in one piece with the pipe stub itself and forms the one end of the pipe stub and has fixation holes arranged in a bolt circle. The pipe boss is in turn fixed to a plate part, which constitutes a part of a subsea structure, for example a connector part or an outboard hub. Therefore the plate part has a circular hole, or opening, formed therein for receipt, support and lead through of the pipe stub itself. Around this opening a number of holes are drilled and tapped and the holes align and correspond with the number of bolt holes in the flange. By mounting the bolts through the holes in the flange, entering them in the tapped holes and tighten up, the pipe boss is fixed to the plate part. This is the traditional way to do this, without that there have been much considerations and reflections about this way to do it.
When the dimensions become large, such as for subsea pipelines, the costs of the materials begin to matter, but also the weight. With a 22 inch pipe boss, i.e. having a diameter of about 560 mm, the needed raw material blank must have a minimum diameter that is at least as big as the largest diameter of the finished machined pipe boss. When the flange was an integrated part of the pipe boss, the diameter of the blank was naturally determined by the outer diameter of the flange, and large volumes of material needed to be machined away. If the pipe boss had to be delivered in SuperDuplex quality, the material costs are substantial. Also the less expensive variant, pipe boss of F65 steel lined with Inconel, will experience substantial material costs, though not so much as for the first mentioned one. The first step to save stock costs was to produce the blank without the intended traditional flange, only with a smaller edge or mini flange. A separate flange ring should then be used to tighten in the pipe boss. This flange ring was often divided in two halves to enable the assembly.
It has, however, proven that the traditional fixation method, and neither the above described new method, has been sufficient with respect to the high forces and bending moments that arises within this kind of couplings. By a closer study, when a traditional pipe boss is subjected to a pure bending moment, one experiences that the force distribution within the pipe flange itself will be distributed by a compression regime in one half and a tension regime in the other half and with a neutral zone in the transition between the halves. In the half subjected to tension, the tension regime will be at maximum in the middle between the neutral zones and gradually decrease toward the neutral zones. This means that only one bolt, or perhaps two, shall take care of the main share of the occurring tensional forces. The tensional forces of the bending moments act against the bolt heads which in turn create tension within the bolts and pulls them till rupture when the bending moments become too high. It limits itself how many bolts you can have, how close they can be placed and which dimensions they can have.
In installations on the seabed, it is unknown which direction the forces may take, i.e. at where in the flange the largest load occurs. Thus it must to be taken into account that the flange is equally good dimensioned all the way round. When the latter method is to be used, i.e. the one with divided locking ring, predominately the bolts on each side of the partition line will be exposed, if the partition line approximately coincides with the spot where the largest load occurs. The bolts near the partition line will be exposed for extreme tensional forces and are snapped off, and then the next ones are pulled off and so on, until the rupture is complete. The forces of the bending moments are in the order of magnitude 3000 kN in addition to a pure tensile load in the pipe boss of 300 kN. Extremely heavy bolts and of especial quality are necessary to manage such tasks, and they cost correspondingly much. In some situations it will neither be possible to manage this within existing regulations.
The now proposed solution has appeared from the idea that it would have been desirable to find a way how to avoid tensional forces in bolts. If such forces could have been replaced with pure compression forces, much of the problem would have been solved. In addition, it would have been time saving not to assemble the many bolts which must have been installed and tightened.